How to delete all faces/geometry that isn't visible? Lowering poly count for unity?
Box selection
Go into edit mode ↹ Tab.
Switch to solid view mode (toggle Z).
Switch to face selection ⎈ Ctrl↹ Tab &gt; F.
Box select the faces. B + drag +  LMB confirm
You have now selected the exterior faces.
Invert the selection. ⎈ CtrlI
You have now selected the interior faces.
Delete the faces ⎈ CtrlX &gt; Faces.